A standards body of mobile communication methods, 3GPP, has recently been deliberating about LTE (Long Term Evolution). Especially for the LTE employing TDD (Time Division Duplex), introduction of AAS (Adaptive Antenna System) has been considered. The AAS changes radio wave directionality by performing adaptive control of weighting on each of a plurality of antenna elements forming an array antenna in accordance with a transmission environment. An adaptive array base station that deals with the AAS, by using, in downlink transmission, an antenna weight calculated based on a reference signal (Reference Signal) transmitted from a mobile station, performs adaptive control such as beam forming, null steering and the like to a desired mobile station.
As illustrated in FIG. 4, a communication frame of the LTE employing the TDD has 10 subframes. Each of the subframes is categorized into a UL subframe for an uplink communication from the mobile station to the base station, a DL subframe for a downlink communication from the base station to the mobile station, and a Special subframe having both a UL area for the uplink communication and a DL area for the downlink communication in the subframe (for example, see Non-Patent Document 1).
Preferably, the AAS defines a pair of (correspondence relationship between) the UL subframe and the DL subframe, and uses the pair for the communication between the base station and the mobile station. This is because, when the base station receives the reference signal in the UL subframe transmitted from the mobile station, the base station may calculate an appropriate transmission weight based on the reference signal and perform the downlink communication with the DL subframe more efficiently.
FIG. 5 is a diagram illustrating a pair of the UL subframe and the DL subframe in detail. A subframe 2 serving as the UL subframe and a subframe 4 serving as the DL subframe are paired with each other and a resource block (radio communication channel) RB6 of each of them is assigned to a common mobile station. The subframe 2 serving as the UL subframe includes DRS (Demodulation Reference Signal) and SRS (Sounding Reference Signal) as symbols for transmitting the reference signal. The DRS is a reference signal transmitted in association with user data from the mobile station to the base station, and the SRS is a reference signal that may be transmitted without being associated with the user data. In a case of FIG. 5, since the subframe 2 serving as the UL subframe is assigned to the mobile station for transmitting the user data to the base station, the mobile station may transmit the reference signal to the base station by using the DRS of the resource block RB6 of the subframe 2. Upon receiving the reference signal, the base station may appropriately perform adaptive array control for the resource block RB6 of the subframe 4.